1. Field of the Invention
The present invention relates to a short dwell coater apparatus for applying coating material onto a web guided by a backing roll.
More particularly, the present invention relates to a short dwell coater apparatus having an upstream overflow and a downstream metering blade.
2. Information Disclosure Statement
In the papermaking art, a web of paper may be coated with various coating materials in order to decrease the porosity of the web or to enhance the printability thereof.
Various coaters have been proposed which generally fall into the following categories. First, on-line coaters include size presses for applying a thin coating of size onto the dried web prior to calendering thereof. Other on-line coaters include coaters for producing so-called "lightweight coated" or LWC paper.
Second, off-line coaters are coaters for applying coating material onto a web supplied from an unwind reel.
One method of coating a web applicable to both of the aforementioned categories includes the application of coating material to the web through a train of counter-rotating rolls. The roll farthest from the web is partially submerged in a pan filled with coating material. Such prior art coaters while generally satisfactory at low speed coating operations, tend to be difficult to control when operating at high speed because the thickness of the coating applied to the web is difficult to control.
Another prior art type fountain coater includes supplying coating material directly to the web supported by a backing roll by means of a cross-machine directional coater trough, or the like. Excess coating material is then subsequently removed from the web by means of a flexible downstream doctor blade.
The aforementioned method of coating a web suffers from the drawback that the coating material tends to soak into the surface of the web dependent on the porosity of the web to be coated, thereby using considerable quantities of the coating material.
More recently, the problems of controllability of the thickness of the coating material and the amount of coating material required for a given length of web were overcome by the introduction of the so-called "Short Dwell Coater", or SDC.
Essentially, the short dwell coater includes an application chamber defined by a housing, the chamber being supplied with pressurized coating material.
The application chamber in the short dwell coater is open to the web which is disposed between the chamber and a backing roll for guiding the web. A metering blade is disposed at the downstream end of the application chamber, and an overflow weir is disposed at the upstream end of the chamber for permitting the overflow of excess coating material metered by the downstream metering blade.
Much success has been experienced with the use of the short dwell coater mainly because the distance between the upstream overflow and the downstream metering blade is relatively short so that the time of application of the coating material to the web is relatively short. Such short dwell of the web during passage past the application zone causes minimal absorption of the coating material into the web, thereby economizing on the amount of coating material required.
Additionally, by regulating the pressure at which the coating material is supplied to the application chamber and by controlling the pressure applied to the metering blade, the thickness or weight of the coating applied to the web may be accurately determined.
However, a problem has been experienced in the use of short dwell coaters when operated at high speeds above 4,500 feet per minute. More specifically, as the machine speed increases above 4,500 feet per minute, there exists a tendency for the web to draw air through the upstream overflow into the application chamber. Such entrained air flows in a direction generally opposite to the direction of flow of the coating material and tends to generate an air vortex within the application chamber.
The air vortex usually rotates in a clockwise direction below the web within the application chamber when the backing roll is rotating in a counter-clockwise direction.
As the coating speed is increased, particularly above 4,500 feet per minute, there exists a tendency for such entrained air to intermittently escape from the application chamber past the metering blade, thereby producing a streaky appearance on the resultant coated web.
The present invention overcomes the aforementioned problem by the provision of means disposed within the application chamber for removing the aforementioned entrained air from the application chamber.
Generally, the present invention includes a perforate conduit extending through the application chamber in a cross-machine direction. The conduit is connected to ambient atmosphere or to a source of partial vacuum. The arrangement is such that the entrained air flows from the air vortex through the perforate conduit away from the application chamber, thereby inhibiting streaking on the resultant coated web.
Therefore, it is a primary objective of the present invention to provide a coater apparatus that overcomes the aforementioned problems associated with the prior art coaters and which makes a considerable contribution to the web coating art.
Another object of the present invention is the provision of a short dwell coater apparatus having a perforate conduit disposed within an application chamber and extending in a cross-machine direction. The conduit permits the flow therethrough of a vortex of entrained air generated within the chamber such that in use of the apparatus, when the vortex of entrained air is generated within the chamber, the air flows through the perforate conduit away from the chamber so that streaking of the coating material due to the intermittent escape of the entrained air past the metering blade is inhibited.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter, taken in conjunction with the annexed drawings.